1. Technical Field
The present invention relates to a fixing device and a guide mechanism included therein, and more particularly, to a fixing device for use in an image forming apparatus, such as a photocopier, facsimile machine, printer, plotter, or multifunctional machine incorporating several of these features, and a mechanism for guiding movement of a rotatable pressure member included in the fixing device.
2. Background Art
In electrophotographic image forming apparatuses, such as photocopiers, facsimile machines, printers, plotters, or multifunctional machines incorporating several of these features, an image is formed by attracting developer or toner particles to a photoconductive surface for subsequent transfer to a recording medium such as a sheet of paper. After transfer, the imaging process is followed by a fixing process using a fixing device, which permanently fixes the toner image in place on the recording medium with heat and pressure.
In general, a fixing device employed in electrophotographic image formation includes a pair of generally cylindrical looped belts or rollers, one being heated for fusing toner (“fuser member”) and the other being pressed against the heated one (“pressure member”), which together form a heated area of contact called a fixing nip. As a recording medium bearing a toner image thereupon enters the fixing nip, heat from the fuser member causes the toner particles to fuse and melt, while pressure between the fuser and pressure members causes the molten toner to set onto the recording medium.
Various methods have been proposed to provide a fast, reliable fixing process that can process a toner image with short warm-up time and first-print time without causing image defects even at high processing speeds.
For example, a known belt-based fixing device employs an endless flexible fuser belt looped into a generally cylindrical configuration, with a stationary fuser pad disposed inside the loop of the belt. Opposite the fuser belt extends a pressure roller that presses against the fuser pad via the belt to form a fixing nip therebetween. The pressure roller is connected with a rotary driver via a gear train, including an output gear and its mating, idle gear, from which torque is transmitted to rotate the pressure roller to in turn rotate the fuser belt in frictional contact with the roller at the fixing nip.
Optionally, the fuser assembly is equipped with a tubular holder of thermally conductive metal, or heat pipe, disposed inside the loop of the fuser belt for heating the fuser belt through conduction. A heater is disposed inside the heat pipe, from which heat is imparted to the entire circumference of the fuser belt looped around the heat pipe. A generally flat, reinforcing plate is provided in contact with the fuser pad to reinforce the fuser pad.
In this fixing device, a releasable biasing mechanism is provided to move the pressure roller away from the fuser belt to release pressure between the pressure roller and the fuser belt. Releasing nip pressure prevents deformation of the fuser belt and the pressure roller, which would occur where the fixing members are continuously subjected to a substantial nip pressure for an extended period of non-operation, while facilitating removal of jammed recording media from between the fuser belt and the pressure roller.
The inventors have recognized that releasing nip pressure through movement of the pressure member, although generally successful for its intended purpose, may create difficulties in the fixing device.
Specifically, one approach to releasing nip pressure is to move the pressure roller away from the fuser belt in a straight direct path along a load direction in which the pressure roller exerts pressure against the fuser belt. Such movement of the pressure roller does not require a substantial space for accommodating the moving roller, while entailing a risk of sudden disengagement of the output gear from the idler gear, which would result in damage and other adverse consequence to the gear train where adjacent gear teeth strike each other during movement of the pressure roller.
Another approach is to move the pressure roller away from the fuser belt in a curved, circumferential path around a given rotational axis. Compared to straight movement, curved movement of the pressure roller can maintain proper engagement between the mating gears, thereby eliminating failure due to interference between gear teeth. However, this approach requires an extensive space for accommodating the moving roller. Moreover, increasing the range of movement of the pressure roller would cause increased interference of the pressure roller with its surrounding structure.